HIV-1 disseminates rapidly in infected individuals. This can at least partially be attributed to virus transfer between T lymphocytes, which occurs very efficiently when infected and uninfected T cells directly contact each other, thus forming the so-called virological synapse (VS). While many features of this transient adhesion structure remain undefined, we know that VS formation necessitates an interaction of the viral envelope glycoprotein (Env), which is expressed at the surface of infected (virus producing) cells, with the viral receptor on uninfected (target) cells. It is not understood, however, why such interactions between Env, which mediates fusion of the viral particle with target cell membranes, and the viral receptor/coreceptor do not routinely result in producer-target cell fusion and thus the formation of a syncytium. Small T cell-based syncytia have been observed in lymph nodes of infected individuals and in humanized mice, but it is evident that most encounters between infected and uninfected cells resolve without fusion. Indeed, fusion-less encounters between these cells are a sine qua non for three central features of HIV-1 pathogenesis: cell-to-cell transmission, killing of bystander cells, and establishment and maintenance of a reservoir of latently infected T lymphocytes. Hence, revealing how fusion between infected and uninfected cells is regulated will not only contribute to our knowledge about how the virus can be efficiently transmitted from cell to cell, it will also aid understanding of key aspects of HIV-1 pathogenesis. We thus propose to investigate how both cell-intrinsic and cell-extrinsic factors, together, facilitate disengagement of producer and target cells upon virus transfer. It will also be tested whether enhancing fusion between the cells beyond the normal level is detrimental to virus dissemination.